Electromagnetic Design of a Low-Fringing-Field Magnetic Bearing Stage for Electron Beam Lithography

Abstract

While electromagnetic actuators have been proven in many precision motion control applications, their stray fields constrain their use in systems that require high stability of charged particle beams. Fringing fields from these actuators should be sufficiently small to have minimal influence on the trajectory and focusing properties of the charged beams. We have built a levitation linear motor that is designed to have significantly reduced fringing fields. The design achieves large far field attenuation because of parallel opposing multipole placement in the synchronous motor bearing. The design is suitable for future generations of electron beam lithography, where the allotted error to stage fringing field may be less than 1nm. The low fringing field design attains very strong fields at the stator, where force is generated, while the fringing fields fall very rapidly. Whereas fields for a dipole fall off as radius-3, the paper presents a prototype synchronous motor magnet array with fields that fall off largely as radius-10 in the far field. The stator has fields that fall off as radius-5. The novel permanent magnet array and coil array technologies can be implemented in stage designs where the fringing field is designed to have negligible influence on the electron beam in a lithography machine. The stator and magnet array have a sufficiently low field to allow placement of the arrays unusually close to the electron beam. This allows a compact, high resonant frequency stage structure. A precision motion control stage based on the low fringing field magnetic bearing is thus shown to be feasible for next generation electron beam lithography.